Fungi can tell us about the origin of sex chromosomes

March 18, 2008 --
Fungi do not have sexes, just so-called mating types. A new study being
published today in the prestigious journal PLoS shows that there are
great similarities between the parts of DNA that determine the sex of
plants and animals and the parts of DNA that determine mating types in
certain fungi. This makes fungi interesting as new model organisms in
studies of the evolutionary development of sex chromosomes.

In
the plant and animal kingdoms there are individuals of different sexes,
that is, bearers of either many tiny sex cells (males) or a few large
ones (females). In the third eukaryote kingdom (organisms with DNA
gathered in the cell nucleus), the fungi kingdom, there are no sexes
but rather a simpler and more primitive system of different so-called
mating types. These are distinguished by different variants of a few
specific genes.

There are many ways to determine sex. In humans it is done by sex
chromosomes. It is thought that this sex difference arose in the plant
and animal kingdom from the simpler system of mating types and that
this happened several times independently of each other throughout
evolution. The change is believed to have happened with the inhibition
of a step in the copying process in DNA, which led to two separate
chromosomes. These then developed further over a long period of time.

"In
humans, sex chromosomes are believed to have developed over the last
300 million years from a common 'proto-sex chromosome,'" says Hanna
Johannesson, who directed the study.

The new study shows for the
first time that even though fungi do not have sexes, there are many
similarities between the parts of the genome that determine sex in
plants and animals and the parts of the genome that control mating
types in certain fungi. The research group specifically studied a spore
sac fungus (Neurospora tetrasperma) and can show that the similarities
are great, regarding both present-day structure and the way in which it
arose.

"It's hard to study the evolution of sex chromosomes,
partly because so many different and important sex-specific characters
are tied to them. But much of this can be avoided if we use simpler
systems, like fungi, as models."